1. Field of the Invention
The present invention relates to a device for limiting the negative output current of an operational amplifier. In other words, the invention relates to a current limiting device associated with a low-side half-stage of an output stage of the operational amplifier. The present invention applies more particularly to an operational amplifier having a differential input stage coupled to a low supply voltage (for example, a negative voltage or ground).
2. Discussion of the Related Art
FIG. 1 is a simplified diagram of an exemplary operational amplifier of the type to which the present invention applies.
The operational amplifier includes a differential input stage 1, a gain stage 2 and an output stage 3.
The differential input stage 1 is biased by a constant current source 4 and the gain stage 2 is biased by a constant current source 5. The output stage 3 is formed by two transistors T1 and T2 defining output high-side and low-side half-stages, respectively, each of which is associated with a respective device 6 and 7 for limiting the output current.
FIG. 1 represents an operational amplifier formed by bipolar transistors. Two diodes D1 and D2 are interposed between source 5 and the gain stage 2 to compensate for the base-emitter voltage drops of the bipolar transistors T1 and T2, of NPN and PNP-type respectively, of output stage 3.
The operational amplifier is supplied between a high voltage Vcc, for example a positive voltage, and a low voltage Vee, for example a negative voltage or ground. The operational amplifier includes two input terminals E- and E+ which are connected to the respective bases of two PNP transistors T3 and T4 of stage 1. The emitters of transistors T3 and T4 are connected to a first terminal of source 4, whose other terminal is connected to the supply line Vcc. The collector of each transistor T3 or T4 is connected to the collector of a respective NPN transistor T5 or T6, whose emitter can be connected, if necessary through a resistor (not shown), to the supply line Vee. The bases of transistors T5 and T6 are interconnected and transistor T5 is diode-connected, i.e., its base is connected to its collector. The collector of transistor T6 forms the output 8 of the differential input stage 1.
Output 8 is connected to the base of an NPN transistor T7 forming, for example, the gain stage 2. The emitter of transistor T7 is connected to line Vee. The collector of transistor T7 forms the output of stage 2. The output of stage 2 is provided to the respective bases of transistors T1 and T2. The collector of transistor T7 is directly connected to the base of transistor T2 which forms the low-side output half-stage of the operational amplifier. The collector of transistor T7 is also connected, through two series diodes D1 and D2, to the base of transistor T1 forming the high-side output half-stage. Transistor T7 is biased by source 5 having a first terminal connected to line Vcc and a second terminal connected, through diodes D1 and D2, to the collector of transistor T7.
The collector of transistor T7 is connected to line Vcc and its emitter is connected, through a resistor R1 of a device for limiting the output current 6, to an output terminal S of the operational amplifier. The collector of transistor T2 is connected to line Vee and its emitter is connected, through a resistor R of a device for limiting the input current 7, to the output terminal S. Resistors R1 and R2 form resistors for measuring the positive and negative currents, respectively, of the output stage 3.
Device 6 is formed by resistor R1 and an NPN transistor T8. The base of transistor T8 is connected to the emitter of transistor T1 and its emitter is connected to terminal S. The collector of transistor T8 is connected to the base of transistor T1.
Device 7 is formed by resistor R2 and a PNP transistor T9. The base of transistor T9 is connected to the emitter of transistor T2 and its emitter is connected to terminal S. The collector of transistor T9 is connected to the bases of transistors T5 and T6 of the differential input stage 1.
The normal operation of an operational amplifier such as represented is FIG. 1 is well known by those skilled in the art and will not be described.
The role of the current limiting devices 6 and 7 is to protect the operational amplifier during its critical operation, i.e., when the output (positive or negative) current of the operational amplifier exceeds the values for which the operational amplifier is designed, risking to damage the operational amplifier. Such a critical operation condition may occur, for example, if the impedance of the load connected to terminal S is too low or in case of accidental short-circuit at the output of the operational amplifier.
When the positive output current of the operational amplifier exceeds a value predetermined by the size of resistor R1, transistor T8 becomes conductive, which causes the base current of transistor T1 to decrease, thereby limiting the positive output current of the operational amplifier. The limit value of the current corresponds to the ratio of the base-emitter junction voltage Vbe.sub.ON of transistor T8 in the conductive state and resistance R1. Under normal operation conditions, i.e., when the current flowing through resistor R1 is lower than Vbe.sub.ON /R1, the base-emitter voltage of transistor T8 is too low and transistor T8 is not conductive.
When the negative output current of the operational amplifier is such that the voltage drop across resistor R2 exceeds the base-emitter voltage Vbe.sub.ON of transistor T9 in the conductive state, transistor T9 becomes conductive, which causes the base current of transistor T6 of the input stage 1 to increase. The resulting increased collector current of transistor T6 causes the base current of transistor T7 to decrease and, thus, the base current of transistor T2 to decrease. As for device 6, the current limitation is determined by the value of resistor R2 and is equal to the ratio of the base-emitter voltage Vbe.sub.ON of transistor T9 in the conductive state and resistance R2. Under normal operation conditions, i.e., when the current flowing through the measurement resistor R2 is lower than Vbe.sub.ON /R2, transistor T9 is not conductive.
A drawback of a conventional operational amplifier such as represented in FIG. 1 is that the negative current limited by device 7 causes oscillations of the output current of the operational amplifier. The limitation current loop, formed by transistor T9 and transistors T6 and T7, is not stable because of the limited bandwidth of PNP transistor T9. This problem does not occur for device 6 since the current limitation loop is formed by an NPN transistor T8 inherently having a wide bandwidth.
Such oscillations are undesirable, especially because they pollute the positive supply line Vcc. For example, if a short-circuit occurs between the output terminal S and the supply voltage Vcc, the oscillations resulting from the current limited by device 7 are reproduced on the supply line Vcc, which may impair the proper operation of other circuits supplied by the supply line Vcc.
Using a PNP transistor for transistor T9 is correlated to the connection of the measurement resistor R2 to the emitter of transistor T2. The conventional device more particularly applies to an operational amplifier whose low-side output half-stage is formed by a so-called vertical transistor, for example, a PNP bipolar transistor whose collector corresponds to the substrate, which makes it necessary to connect the measurement resistor R2 to the emitter of transistor T2.
A further drawback is that the action of device 7 on the differential input stage 1 is asymmetric, regardless of whether transistor T2 is vertical or lateral, which creates an offset in the biasing of input transistors T3 and T4 of the operational amplifier. The conventional limitation of the negative output current causes a current to be injected in the bases of transistors T5 and T6, and thus in the collector of transistor T5 only.